Specifies the reference value that incoming alpha values are compared to.
This value is clamped to the range [0, 1],
where 0 represents the lowest possible alpha value
and 1 the highest possible value.
The initial reference value is 0.

DESCRIPTION

The alpha test discards fragments depending on the outcome of a comparison
between an incoming fragment's alpha value and a constant reference value.
%f3glAlphaFuncfP specifies the reference value and the comparison function.
The comparison is performed only if alpha testing is enabled. By
default, it is not enabled.
(See
%f3glEnablefP and %f3glDisablefP of %f3GL_ALPHA_TESTfP.)
f2funcfP and f2reffP specify the conditions under which
the pixel is drawn.
The incoming alpha value is compared to f2reffP
using the function specified by f2funcfP.
If the value passes the comparison,
the incoming fragment is drawn
if it also passes subsequent stencil and depth buffer tests.
If the value fails the comparison,
no change is made to the frame buffer at that pixel location. The
comparison functions are as follows:

%f3GL_NEVERfP

Never passes.

%f3GL_LESSfP

Passes if the incoming alpha value is less than the reference value.

%f3GL_EQUALfP

Passes if the incoming alpha value is equal to the reference value.

%f3GL_LEQUALfP

Passes if the incoming alpha value is less than or equal to the reference value.

%f3GL_GREATERfP

Passes if the incoming alpha value is greater than the reference value.

%f3GL_NOTEQUALfP

Passes if the incoming alpha value is not equal to the reference value.

%f3GL_GEQUALfP

Passes if the incoming alpha value is greater than or equal to
the reference value.

%f3GL_ALWAYSfP

Always passes (initial value).
%f3glAlphaFuncfP operates on all pixel write operations,
including those resulting from the scan conversion of points,
lines,
polygons,
and bitmaps,
and from pixel draw and copy operations.
%f3glAlphaFuncfP does not affect screen clear operations.

NOTES

Alpha testing is performed only in RGBA mode.

ERRORS

%f3GL_INVALID_ENUMfP is generated if f2funcfP is not an accepted value.
%f3GL_INVALID_OPERATIONfP is generated if %f3glAlphaFuncfP
is executed between the execution of %f3glBeginfP
and the corresponding execution of %f3glEndfP.